The present invention relates to a power amplifier, in particular, to a power amplifier that amplifies and composites differential signals.
At a mobile phone terminal or wireless LAN terminal, it is necessary for a transmitter, that is, a power amplifier, to transmit power of 10 to 30 dBm at maximum. Recently, a technique is proposed, which composites differential signals amplified by a pair of transistors with a transformer using a silicon MOS type transistor (insulating gate field effect transistor) in a power amplifier having signal frequency of about 1 GHz to 6 GHz. Such a power amplifier is shown in FIG. 11.9.2 in Non-Patent Document 1 (Jongchan Kang, et. al. “A Single-Chip Linear CMOS Power Amplifier for 2.4 GHz WLAN” ISSCC (International Solid-State Circuits Conference) 2006, p 208-209, 649.), and hereinafter referred to as a transformer type power amplifier of one differential pair.
In the power amplifier described in Non-Patent Document 1, a primary coil (a coil is referred to as a slab in Non-Patent Document 1) and a secondary coil magnetically coupled with the primary coil are arranged in parallel with each other. The middle point of the primary coil is a terminal to which a power supply voltage is supplied and by coupling a capacitor to the middle point, it is possible to suppress second harmonics.
As a technique to make an attempt to further increase output more than the transformer type power amplifier of one differential pair, it is proposed to composite differential amplified signals from two or more pairs of transistors with a transformer. Such a power amplifier is described in Patent Document 1 (Published Japanese translation of PCT patent application No. 2005-503679) as a distributed circular geometry power amplifier and hereinafter referred to as a transformer type power amplifier of multiple differential pairs. In Patent Document 1, a configuration is disclosed, in which differential signals respectively amplified by four pairs of transistors are composited with the secondary coil of the transformer coupled in series.
As another technique to make an attempt to further improve efficiency more than the transformer type power amplifier of one differential pair described in Non-Patent Document 1, a configuration is disclosed in Patent Document 2 (Japanese Patent Laid-Open No. 2006-295896), in which the differential signals of two pairs of transistors are composited with a transformer where primary coils having different shapes are arranged on both sides of a secondary coil.